Vibrational study of isolated 18-crown-6 ether complexes with alkaline-earth metal cations

Laser infrared multiple photon dissociation (IRMPD) spectroscopy has been employed to probe the C-O and C-C stretching vibrational modes of 18-crown-6 ether (18c6) complexes with alkaline-earth metals (Mg2+, Ca2+, Sr2+ and Ba2+) stored in the cell of a Fourier Transform Ion Cyclotron Resonance mass spectrometer. Computations at the B3LYP/6-311++G(2d,2p) and B3LYP/aug-cc-pVDZ levels of theory agree well with the most salient features of the experimental spectra and allow to characterize the lower energy conformers for each type of complex. A pronounced shift of the C-O stretching band, but not of the C-C band, is found in comparison to the similar IRMPD spectra reported previously for the 18c6 complexes with alkali metals. This is attributed to the tighter coordination and stronger binding of the divalent alkaline-earth cations to the oxygen sites, and to the degree of folding of the crown ether backbone. Nevertheless, the conformational landscape and symmetry constraints of the complexes follow a pattern similar to that found for the alkali metal cations. The most stable conformers evolve from compact D2 geometries for the smaller cations, Mg2+ and Ca2+, to more open C2 configurations for Sr2+ and to a planar D3d structure for Ba2+